add set cp-abi command

[Eli Zaretskii]

> Date: Wed, 10 Apr 2002 11:14:13 -0700
> From: Jim Ingham <jingham@apple.com>
> 
> > Please explain in a few words what "ABI" means; I don't think Joe Random
> > C++ User will necessarily know that.  A list of possible values one can
> > use (in addition to "auto") is also something the manual should have.
> 
> I not sure I agree with this.  First off, this "set" section doesn't 
> explain what other similarly obscure terms mean (like "set vtable").  
> These are expert features, so if you need them presumably you know what 
> they are.

Nevertheless, I think it doesn't hurt to say something like this:

  Set the C@t{++} @dfn{ABI} (Application Binary Interface) that
  @value{GDBN} will use to decode C@t{++} objects.

> Also, the available types are dynamically generated.

Does it mean we don't know at least some of the possible values in
advance?  I'd be surprised.